lgA nephropathy (IgAN) is the leading cause of glomerulonephrits in the world. Prominent deposition of IgA1 in the mesangium is the hallmark of the disease. Despite over 25 years of investigation, the reason why lgA localizes to the mesangium and the relationship between this deposition and slow but progressive development of renal dysfunction in some patients with IgAN remains unknown. In this proposal, we will test the general hypothesis that in lgAN, abnormalities in the structure of the circulating lgA1 cause a disproportionate fraction of the lgA to bypass the normal catabolic pathway in the liver and/or to bind selectively to the mesangial cells in the kidney. The project will focus on the O-linked oligosaccharides bound the hinge region peptide of lgA1, since these are the structures that distinguish lgA1 from lgA2 and the other major Igs isotypes. The binding of lgA1 to two cellular receptors, the asialoglycoprotein receptor on cultured hepatoma cells and the FcalphaR receptor of primary cultures of mesangial cells will be examined in detail. These receptors were chosen from a number of lgA receptors, since they are most likely to be involved in an lgA catabolic defect (ASGPR) or in targeting of the abnormal lgA to the kidney (mesangial FcalphaR). Both receptors require oligosaccharides on their ligands for binding. Three specific hypotheses will be tested: 1) that the lgA1 from some patients with IgAN will bind relatively better to the FeaR than the ASGPR, 2) that binding of the lgA1 from some patients with lgAN induces the expression of more FcalphaR and the production of cytokines and growth factors from cultured mesangial cells, and 3) that some of the functional abnormalities defined in l and 2 can be attributed to aberrant glycosylation of the hinge region of lgA1. The methods to be used in this study include isolation by lectin affinity chromatography of lgA1 from the serum of 20 patients with lgAN, 10 normals subjects and 10 patients with other forms of glomerulonephritis. Detailed studies of the binding, internalization and the intracellular fate of the selected lgA1 preparations will be undertaken. The effect of various preparations of lgA1 on FcalphaR expression will be assessed by Northern blotting and release of transforming growth factor, platelet derived growth factor and lL-6 by immunoassays. Carbohydrated structural analysis will be performed by beta elimination and HPLC, lectin binding and enzymatic degradation. The hinge glycoproteins produced with lgA-specific proteases and trypsin will be submitted for aa sequence analysis. Successful completion of these studies should identify one or more abnormalities of lgA structure and function predispose some patients with lgAN to mesangial deposition of lgA. These studies should eventually allow identification of patients at risk for development or progression of lgAN. Specific sites for therapeutic interventions may also be suggested by these studies.